10 September 2018
Frozen debris lobes: an interesting hazard in Alaska
The Anchorage Daily News has a nice article about the hazards posed by, and mitigation enacted to deal with, frozen debris lobes on the Dalton Highway in the icy north of Alaska. In this case, the movement of the frozen debris lobes towards the road pose an increasing threat of significant disruption. The Alaska Department of Transportation and Public Facilities has now spent about US$2 million rerouting the highway to manage the hazard for the next few years.
Frozen debris lobes are creeping landslides formed from rocks, soil, vegetation and water. The University of Alaska Fairbanks has a good project site about these landslides; they note that in some cases in the vicinity of the road they are moving at 1.7 cm per day. They include the following cross-section to illustrate the structure of these landslides:-
The concept is that, although these landslides are located in frozen soils, the depth of the slide provides sufficient pressure that liquid water is present at the base. This allows the landslide to creep. Unfortunately, as temperatures warm the availability of water in increasing, so the rate of movement is accelerating.
The University of Alaska Fairbanks team have identified 43 frozen debris lobes, 23 of which are uphill of the highway within 1.6 km of the road. The Google Earth image below shows some of these landslides, including one (known as the blob) that was very close to the highway:-
The new highway is expected to be safe from the blob for the next 20 years or so. Interesting, the old road has been left in place so the the research team can investigate what happens when the frozen debris lobe starts to overrun it.